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Binding and Mobility of Atomically Resolved Cobalt Clusters on Molybdenum Disulfide S. A. Kandel and P. S. Weiss* Department of Chemistry, PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802-6300 ReceiVed: April 4, 2001
We present the results of an ultrahigh vacuum, low-temperature scanning tunneling microscopy investigation of cobalt clusters adsorbed on the surface of single-crystal molybdenum disulfide. This system is of interest as a model for understanding industrial hydrotreating catalysts. We observe small metal clusters that bind on the MoS2 basal plane, and determine from atomically resolved images that these clusters are composed of cobalt atoms bound exclusively atop surface sulfur atoms. Small clusters are also observed nucleated near surface defects. Larger cobalt clusters are observed primarily bound at MoS2 surface defects, and are additionally observed near and on surface steps. Cobalt clusters on the MoS2 surface are extremely labile and change their size and shape on the time scale of imaging. Individual atoms are observed to adjoin to and detach from clusters, possibly under the influence of the probe tip; under appropriate tunneling conditions, the tip will sweep all clusters from a scanned area. The low barrier to diffusion for adsorbed cobalt on MoS2 is in accord with earlier measurements of nickel on MoS2, and supports conclusions drawn about the action of promoter species in hydroprocessing catalysis.
Hydroprocessing reactions, in which sulfur- and nitrogencontaining functional groups are removed from hydrocarbons, are of paramount importance in industrial petroleum processing. The most commonly employed catalysts consist of molybdenum disulfide (MoS2) crystallites dispersed on an alumina support, along with added transition-metal (typically nickel or cobalt) promoters.1,2 Hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) take place on these catalysts at elevated temperatures in a hydrogen atmosphere. It is well accepted that reaction occurs at the MoS2 edge sites, which are decorated with the transition-metal promoters,2-6 although it has additionally been determined that the dispersion (ratio of edge to basal plane sites) influences reactivity.6 The system is highly heterogeneous, and this heterogeneity plays a significant role in its catalytic properties. We seek to understand hydroprocessing on the atomic level by studying a model catalyst consisting of a MoS2 single-crystal surface upon which a small quantity of cobalt has been deposited. We investigate the structure and dynamics of this model system through scanning tunneling microscopy in ultrahigh vacuum at 4 K. Previous work in this laboratory on MoS2 showed that nickel atoms retained mobility on the surface even at 77 K,7,8 with thermal diffusion rates so high as to prevent any imaging of adsorbed metal. The high mobility of the adsorbed nickel allows clustering on the surface, and at 4 K, nickel atoms and small clusters were observed. Scanning tunneling spectroscopy (STS) of a Ni3 cluster showed it to be well suited for binding nucleophilic sulfur-containing or nitrogencontaining molecules. On the basis of our observations, we proposed a new mechanism for promoters that could play a role in HDS or HDN processes, in which mobile metal atoms or clusters first bind reagent molecules and then transport them to active edge sites. The work presented in this article extends these studies with the observation of clusters of cobalt on MoS2 basal planes, defect sites, and step edges. * Author to whom correspondence should be addressed.
The experimental apparatus has been described in detail elsewhere, and consists of a scanning tunneling microscope (STM) of the Besocke design housed in an ultrahigh vacuum chamber with a base pressure below 1 × 10-10 Torr at room temperature.9 The STM hangs in a cryostat below the vacuum chamber and is maintained at 4 K by immersion in liquid helium. Before imaging, the STM tip was first cleaned and sharpened by field emission as well as brief controlled contacts with a Au{111} surface. The MoS2 used was natural molybdenite10 and was freshly cleaved to expose a clean surface prior to introduction into the experimental chamber. Once in vacuum, the surface was dosed with cobalt by exposure to a heated cobalt filament at a distance of roughly 5 cm. The sample was then transferred (in vacuum) to the STM and allowed to equilibrate thermally at 4 K for 12 to 24 h before imaging. After imaging was completed, the sample was re-cleaved (in air) to remove adsorbed metal before the next experiment. While natural impurities in the MoS2 can alter the material’s electronic properties, we observed similar behavior for the adsorbed cobalt on several different substrates. All STM images are recorded in constant-current feedback mode, with the bias voltage applied to the sample. The images shown are processed only with a constant offset for each line to remove noise perpendicular to the scanning direction; excepting the Fourier analysis used in the binding-site determination for Figure 1, no other filtering has been performed. After depositing cobalt on MoS2, we observe cobalt clusters and evidence of significant mobility of these adsorbed species. Figure 1 (a) shows an STM image of a small cobalt cluster. The internal atomic structure of the cluster can be resolved; additionally, the underlying hexagonal surface lattice can be discerned clearly. We utilized the resolution of both of these features to determine the binding geometry of the cluster to the surface; this has been presented previously by Doering et al. for benzene adsorbed on copper and nickel surfaces.11 To
10.1021/jp011264q CCC: $20.00 © 2001 American Chemical Society Published on Web 08/02/2001
Cobalt Clusters on Molybdenum Disulfide
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Figure 1. (a) Constant-current STM image of a three-atom cobalt cluster adsorbed on MoS2; (50 Å)2 image under +2.5 V sample bias and 0.1 nA tunneling current. (b) Fourier processed image in which the underlying lattice has been extracted and then added into the image to enhance contrast. The image is a (25 Å)2 zoom of panel (a). The cobalt atoms (bright orange/red) are situated in 3-fold hollow sites between exposed sulfur atoms (dark blue).
facilitate the binding site determination, we applied a 2-D Fourier transform to the image, and filtered in frequency space to isolate the MoS2 lattice. This process resulted in a “clean lattice” which could be overlaid upon the original image. A portion of this lattice was then added into the data in Figure 1 (a) to increase the image contrast. The resulting processed image, shown in Figure 1 (b), shows the cluster as clearly atomically resolved. We believe this cluster consists of three cobalt atoms, although the noise along the STM scanning direction creates some ambiguity in this assignment, and the scanning noise evident (especially for the cobalt atom nearer the bottom of the image) may indicate an additional component atom or some motion on the time scale of imaging. It is now generally believed that STM images of the MoS2 surface usually reveal sulfur atoms,12-14 which appear as protrusions in the image in Figure 1. In the processed image, examination of the position of the cluster atoms relative to the surface lattice shows that each cobalt atom is adsorbed on top of a surface sulfur site. A theoretical study of features in MoS2 images has suggested that the contrast mechanism in STM images can depend on the tunneling conditions;13 we observe that the appearance of the MoS2 lattice can change quite markedly as a function of tip state. Consequently, our assignment of protrusions as S atoms is tentative. However, it is clear that the component atoms of the cobalt cluster in Figure 1 are entirely commensurate with the underlying MoS2 lattice. While small clusters such as this one are observed on the surface, larger cobalt clusters are typically situated at defect sites. This is illustrated in Figure 2, in which panels (a)-(c) show the same (250 Å)2 area imaged successively over 15 min. In these images, cobalt clusters appear as protrusions, while surface defects are imaged as depressions. All of the clusters observed in this area are associated with surface defects; indeed, it is rare to observe a large cluster that is not situated in or near a defect site. Furthermore, by comparing sequentially recorded images, we can see that the size and shape of the clusters change over time, and several examples of this are indicated with arrows. For example, the two-lobed cluster in the bottom left portion of the image loses its top lobe after panel (a) is recorded,
and the cluster right of the image center is cut in half in panel (b), but regains the lost cobalt in panel (c). The mobility shown in the rearrangement of these clusters is likely (at least in part) induced by the STM tip; however, the relative ease with which these metal clusters are torn apart and reformed indicates that the barriers to diffusion across the surface and the cohesive energies of the clusters are quite small. We attribute the streakiness in these images as due to cobalt atoms entrained underneath the scanning STM tip, which we also observed for nickel on MoS2.7,8 Additional evidence of weak cluster binding is obtained by lowering the bias voltage slightly, after which we find that nearly all the cobalt clusters can be swept away in the process of scanning. After recording the image shown in panel (c), we rastered the STM tip over the same area at this reduced sample bias voltage (1.5 V). The larger (500 Å)2 image shown in panel (d) was then recorded, centered on the same area recorded in panels (a)-(c), indicated by the dashed square. The cobalt clusters seen in the prior images are gone; in the center of the image, only the underlying defects remain. At higher resolution, we can observe individual cobalt atoms coalescing onto or detaching from a cluster. Figure 3 shows sequentially recorded images of a small cobalt cluster situated near a surface defect. In panels (a) through (g), the size of the cluster varies from 2 to 9 atoms, as determined by atomically resolved images. The images are recorded with the tip rastering quickly along different directions, and we see a correspondence between this “fast scan” direction and the locations at which atoms in the cluster either attach or detach. For example, panel (b) shows a “c-shaped” cluster imaged while scanning left to right. While scanning from bottom to top in the image in panel (c), one atom annexes onto the bottom side of the defect while another atom detaches from the top. Similarly, scanning from right to left in panels (e) and (f) causes most of the atoms in the cluster to be swept away from the defect site. The MoS2 crystal cleaves to produce large, atomically flat terraces; however, occasionally we were able to observe MoS2 steps, as well as cobalt clusters adsorbed near and on these steps. We image naturally occurring steps on the MoS2 substrate, as
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Figure 2. (a)-(c) (250 Å)2 images of larger cobalt clusters on MoS2; sample bias is +2 V, current is 0.1 nA, and the image is acquired scanning the tip from bottom left to top right. The clusters are imaged as protrusions (white) and are observed settled in surface defects, which are imaged as depressions (black). The images are taken sequentially, with 3 to 5 min elapsing between each image. Several of the clusters are observed to change size and shape between images; two of these clusters are indicated in panel (a) by arrows. (d) (500 Å)2 image taken centered on the same area shown in (a)-(c), after that area was scanned at +1.5 V bias. All of the cobalt clusters in the central area have been swept away, leaving only the underlying defects.
opposed to the edges of MoS2 nanocrystallites formed by Besenbacher, Topsøe, and co-workers.15,16 In contrast to the highly resolved images of these nanocrystallites, we find it difficult to maintain high resolution near natural steps. Images of cobalt clusters on natural MoS2 steps are shown in Figure 4. The clusters and steps are most easily visualized in the threedimensional rendering shown in Figure 4 (a), which shows a (350 Å)2 area that includes a single step adjacent to a double step. A small cluster abuts the bottom of the double step, and a larger cluster appears imbedded near the top of the step; a third cluster is near the step on the top terrace. All three clusters are of similar topographic height as measured by the STM. Panel (b) shows a topographic image of the area rendered in panel (a). In panel (c), a different region is shown, in which the cluster adsorbed at the top of the step exhibits a streakiness similar to that observed for the mobile clusters displayed in Figure 2; this implies that binding of the clusters at the MoS2 steps is also relatively weak, and that the mobility of cobalt on the surface at higher temperatures may allow clusters to form at and dissipate from step edges. We observe that the steps are mostly clean, with relatively few bound clusters. This provides additional evidence that metal does not bind irreversibly at step edges, either during the room-temperature deposition or during cooling to 4 K. This STM investigation of cobalt adsorbed on MoS2 has revealed that adsorbed cobalt, like nickel, is extremely mobile
and is readily translated across the surface under the influence of the STM tip. We have observed atomically resolved small cobalt clusters both on the bare surface and nucleated near surface defects. On the defect-free surface, cobalt atoms in clusters bind exclusively at 3-fold hollow sites between surface sulfur atoms. We additionally observe single atoms detaching from and adhering to these clusters on the time scale of imaging. Large clusters almost exclusively settle in defects and are observed to change their shape and size over time during successive imaging. Cobalt clusters also settle on and near MoS2 steps and can show similar mobility. In our previous study of nickel adsorbed on MoS2, we used bias-dependent STM images to show that nickel clusters showed enhanced densities of empty states (and depleted densities of filled states) relative to the MoS2 surface.7,8 While the basal planes of the surface have fully saturated bonding and are consequently inert, we posit that adsorbed metal clusters would provide binding sites for electron-donating sulfur- and nitrogencontaining molecules. The measurements reported here, as well as our previous measurements, show that atoms and clusters have exceptionally low barriers to diffusion on the surface, and are almost certainly mobile at higher temperatures. We have proposed that one of the roles of the transition-metal promoters in HDS and HDN is to bind reagents and then mobilize them on the surface until they can reach the catalytically active edge sites.
Cobalt Clusters on Molybdenum Disulfide
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Figure 3. Seven images of a (50 Å)2 area at +2.5 V sample bias and 0.1 nA tunneling current, taken sequentially over approximately 20 min. The cluster (protrusion, shown as bright) nucleates about the surface defect (depression, shown as dark) and can be seen to vary dramatically in shape as it is imaged. The scanning direction of the STM tip is indicated by the arrow to the lower left of each image. Noise along the fast scan direction may be due to cobalt atoms entrained underneath the scanning tip.
Figure 4. Images of MoS2 steps with adsorbed cobalt clusters. Imaging conditions are +2.0 V sample bias and 0.1 nA tunneling current. (a) Three-dimensional rendering of a (350 Å)2 area including one single step (lower) and one double step (higher). Three clusters are adsorbed near or on the steps and are marked on the figure. (b) Topographic image of the same area rendered in panel (a). (c) (400 Å)2 area including a single step. The cluster adsorbed in the upper left corner of the image is streaky and noisy, which may indicate motion of the cluster during acquisition of the image.
We do not purport to apply directly the measurements upon our model system to the actual catalytic system used in hydroprocessing. Our experiments are performed on singlecrystal MoS2 and nonsulfided metal at low temperature and
ultrahigh vacuum; this is substantially removed from the hightemperature, high-pressure, partially sulfided conditions used in commercial HDS and HDN. However, the observed mobility of atoms and clusters observed in our investigations have led
8106 J. Phys. Chem. B, Vol. 105, No. 34, 2001 us to propose this additional mechanism of promotion, which we believe could be qualitatively useful in understanding this complicated catalytic system. In our previous investigations, we were only able to image isolated nickel atoms moving on the MoS2 basal plane, and it was not entirely clear whether metal atoms mobile on the surface would bind irreversibly to step edges, rendering them useless for repeated transport of reagents to these sites. The current study additionally reports high tip-induced mobility of clusters over defect sites (where they are presumably more strongly bound than on defect-free terraces). Furthermore, we observe small clusters bound at and near the steps, and show that at least some of these clusters continue to demonstrate mobility during STM imaging. In conclusion, we have observed cobalt clusters of various sizes on the MoS2 surface. Cobalt is highly labile on the surface. The observed mobility is due (at least in part) to the influence of the STM tip; however, mobility is observed under all tunneling conditions and is indicative of an extremely low barrier to diffusion. Diffusion of cobalt is not stabilized by intracluster binding; larger cobalt clusters are observed to change their shape and size with successive imaging. The atomically resolved 3-atom cluster in Figure 1 indicates that the component cobalt atoms are bound exclusively atop surface sulfurs. Consequently, we observe the Co-Co bonds to be highly extended, which is in accordance the lack of cohesion observed for larger cobalt clusters, as well as the near-linear geometry of the three-atom cluster presented. All of these factors indicate that the interaction between adsorbed cobalt and the MoS2 substrate is strong. The interaction potential is only weakly corrugated, however, resulting in commensurate binding at 4 K, but resulting in facile manipulation and motion, at least along some lattice directions. These conclusions have implications for the formation and growth of metal particles on sulfur surfaces as well as bimetallic sulfide clusters. Besenbacher, Topsøe, and co-workers observed MoS2 and Co/MoS2 nanocrystallites formed on a Au{111} substrate.15,16 These nanocrystalltes are triangular (or hexagonal for Co/MoS2) and aligned along gold crystallographic directions; this is in agreement with our conclusions regarding strong interactions between metals and MoS2 surfaces. It is possible that the interaction of gold with MoS2 is weaker than that of cobalt; however, it is interesting to consider the possibility that the
Kandel and Weiss interaction of the nanocluster with the underlying substrate could perturb the geometry of the cluster or its edges. Future investigations of this reaction will entail a comparison of the electronic properties and mobility of different adsorbed metal species on MoS2, as we hope to be able to correlate known catalytic activity of several promoter species with our observations at the atomic level. We additionally plan to adsorb model HDS and HDN reagents such as thiophene (C4H4S) and pyridine (C6H5N) onto the adsorbed metal clusters in order to study molecule/metal cluster complex mobility. Acknowledgment. We acknowledge Dr. Kevin F. Kelly for helpful discussions related to image processing and analysis. This work was supported by the National Science Foundation, the Office of Naval Research, the Petroleum Research Fund administered by the American Chemical Society, and the Exxon Education Foundation. References and Notes (1) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley and Sons: New York, 1994. (2) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis, Science and Technology; Springer-Verlag: Berlin, 1996. (3) Topsøe, H.; Clausen, B. S.; Topsøe, N.-Y.; Pederson, E. Ind. Eng. Chem. Fundam. 1986, 25, 25-36. (4) Chianelli, R. R.; Daage, M.; Ledoux, M. J. AdV. Catal. 1994, 40, 177-232. (5) Farias, M. H.; Gellman, A. J.; Somorjai, G. A.; Wold, A.; Chianelli, R. R.; Liang, K. S. Surf. Sci. 1984, 140, 181-196. (6) Daage, M.; Chianelli, R. R. J. Catal. 1994, 149, 414-427. (7) Kushmerick, J. G.; Kandel, S. A.; Han, P.; Johnson, J. A.; Weiss, P. S. J. Phys. Chem. B 2000, 104, 2980-2988. (8) Kushmerick, J. G.; Weiss, P. S. J. Phys. Chem. B 1998, 102, 10094-10097. (9) Ferris, J. H.; Kushmerick, J. G.; Johnson, J. A.; Youngquist, M. G. Y.; Kessinger, R. B.; Kingsbury, H. W.; Weiss, P. S. ReV. Sci. Instrum. 1998, 69, 2691-2695. (10) The molybdenite samples were mined in Ontario, CA, and obtained from Nature’s Window, Wyomissing, PA. (11) Doering, M.; Rust, H.-P.; Briner, B. G.; Bradshaw, A. M. Surf. Sci. Lett. 1998, 410, L736-L740. (12) Magonov, S. N.; Whangbo, M. H. AdV. Mater. 1994, 6, 355-371. (13) Altibeli, A.; Joachim, C.; Sautet, P. Surf. Sci. 1996, 367, 209220. (14) Whangbo, M. H.; Ren, J.; Magonov, S. N.; Bengel, H.; Parkinson, B. A.; Suna, A. Surf. Sci. 1995, 326, 311-326. (15) Helveg, S.; Lauritsen, J. V.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J. K.; Clausen, B. S.; Topsøe, H.; Besenbacher, F. Phys. ReV. Lett. 2000, 84, 951-954. (16) Lauritsen, J. V.; Helveg, S.; Lægsgaard, E.; Stensgaard, I.; Clausen, B. S.; Topsøe, H.; Besenbacher, F. J. Catal. 2001, 197, 1-5.
